But where is the documentation? Obviously you added those diodes for greater protection, so it's pertinent that documentation explains what pitfalls to avoid when using the uCurrent. Surely someone must have put together some documentation on the uCurrent by now, but I am not finding it. Do you have access to that?

But where is the documentation? Obviously you added those diodes for greater protection, so it's pertinent that documentation explains what pitfalls to avoid when using the uCurrent. Surely someone must have put together some documentation on the uCurrent by now, but I am not finding it. Do you have access to that?

It is a really nice precision current to voltage converter, I have one and I can recommend it. It is obvious how to use it, but looks like you are right, I can't find any beginners documentation either. And there are some pitfalls, for example when using the nA range you have to take care that it doesn't oscillate which can happen with long cables, as I've discovered:

Intuitively, it seems obvious how to use it, but that would be true for the ON switch position. But for the SHORT position, I am still not clear on that.

I only measure DC, from 0V to a max of about 33V. I have a $2000 6.5 digit Fluke 8845A and it works great to measure current down to about 1uA, but the nA range is terrible. When the leads are floating I see 23nA on the meter. So I want accurate nA measurements, especially to check low power designs with PIC MCUs.

I've also been reading about button cell batteries dying in 60 seconds. It's not clear if people are using new or old cells. Does the kit come with the AAA battery holder? Even so, I've been reading that increasing the voltage will increase sensitivity and noise too, and I am not sure if I want to deal with that. Your capacitor fix might work though.

The short position does what the name says, it shorts the input shunt, so that there is 0 V burden voltage for your circuit (of course, you'll have 0 V output voltage in this position, too).

I guess measuring nA is tricky, no matter what instrument you use.

I think my uCurrent came with the AAA battery holder, I just had to solder it. The CR2032 coin cell battery I used before lasted hours and was still the first battery when I replaced it with the battery holder for greater output voltage and even longer battery life. But I don't need it often, I guess I'll have to replace the batteries in some years now

The short position does what the name says, it shorts the input shunt, so that there is 0 V burden voltage for your circuit (of course, you'll have 0 V output voltage in this position, too).

I see... You can leave uCurrent connected in the circuit, and sliding the the switch to the SHORT position will make it as if uCurrent were not connected to the circuit at all.

Well, thank you for the advice. It seems that replacing R4 to reduce LED brightness (or remote the LED altogether), combined with the 3 AAA battery pack mod would be prudent for everyone. I will consider both. Not sure about the dual Schottky diode mod that another use reported though. It seems to only help if the uCurrent is OFF, but I would want protection when it's ON. I can't tell you how many times I've blown those stupidly expensive 400mA fuses in the Fluke Benchtop meter!

Right, the schottky diode mod only protects the OpAmp inputs when it is in off position. I don't know if this is necessary, even if voltage is applied to the inputs. And usually it is always on anyway when I use it, so this wouldn't be a problem.

But it has no over current protection, it is intended for sub 1A measurements and where you know it can't be higher, e.g. because you use a power supply with current limit. And you can easily destroy the 10 ohm resistor in the uA range. E.g. apply 5 V, which results in 500 mA, so 2.5 W for the poor little 0805 resistor and the magic smoke will escape. Or you could destroy the OpAmp, if you apply a too high voltage with the nA range instead of measuring the current in series, because the 10 k shunt resistor would survive it, but unlikely that the OpAmp survives it, e.g. if you short accidentally 30 V. For the mA range I think you would need a lot of amps to vaporize the 0.01 ohm shunt, because the cables etc. might absorb most of it. The switch might blow up first in this case.

I guess most of it could be solved with a fuse and some big diodes in parallel to the shunts, but this might affect the accuracy.

PS: I think Dave's upcoming new multimeter will have a nA range, too, and of course all the proper input protections. But if you have already a Fluke 8845A, you probably don't need it, and just the uCurrent would be less expensive.

The "SHORT" setting allows you to put the uCurrent into the circuit with the end result that it is actually not in the circuit insofar as 50mA or even 1A could flow through your tapped test point without problem or blowing up the uCurrent. So is it better to start by putting the uCurrent into SHORT mode and then attached it to the circuit (with power off, of course), and then switch to ON (with the appropriate Amperage range selected, of course) to start testing? Or is it always best to start with the uCurrent OFF and then connect it and then switch to ON?

Honestly, I've accidentally blown about 5 expensive 400mA fuses on my Fluke 8845A through the years, and that was when I was being cautious. So without any fusing at all, and in light of the price point on this little uCurrent gadget, I would like to be darned sure that the way I use it is not going to fry my investment for a very long time, if ever.

I don't think it matters much, but sounds good to keep it in short setting first. I have to admit, I never blew up a fuse in one of my multimeters so far, but I'm just a programmer and do some retro electronics and digital electronics with microcontrollers etc., so currents are low most of the time, and additionally limited by the power supply. If you blow up your fuse often, you might add a fuse to the uCurrent, too. Dave describes a typical circuit for it here:

You would need only the 600 mA fuse and a big rectifier after it, or two big anti-parallel diodes. I think this will protect it from over-current and over-voltage (to some degree, don't short 220 V mains with it). Might be difficult to mount it inside. Maybe the fuse inline with one measurement cable, and then the diodes soldering to the terminals on the back? But I have no idea what diodes you can use and how it affects the measurement. I guess it is not that much of a problem, because the leakage current of diodes are very low compared to the shunt resistors? And might be good to use a "fast" fuse.

He mentioned his new multimeter a few times, e.g. in this video:

You can read all the details about it in the Supporters Lounge (you get access to this exclusive club as a Patreon), but he doesn't want it to be public at this time.

Thank you, Frank. Very informative. But yet more reason to have that Fire Sale I proposed, clear out old uCurrent Gold inventory, and then offer an updated uCurrent with all the major hacks people all incorporation (protection, reduce LED brightness, use a 3-AA battery pack by default, etc). That mod of the uCurrent probably debut faster than a complex multi-meter built from the ground up.

By the way...

While Googling on the topic of nA measurements, I came across the following page and found the comments section of interest:

Burden Voltage is the voltage produced across a DVM’s internal current shut when it is in Amperes mode. Burden Voltage is named incorrectly. The burden units are strictly V/A (the voltage produced across the shunt for a given amount of current going through it). Units V/A equal Ohms, which is the resistance of the current shunt. So the strictly correct term is Burden Resistance, but the incorrect term Burden Voltage is now simply accepted as the historical norm.

Interesting idea, but burden voltage, or whatever you want to call it, is too high for many measurements. E.g. for 60 nA and 10 megohm input resistance it is already 0.6 V. Too much if you want to check low voltage / low power microcontrollers. But it could be useful if you want to measure below 10 nA. But I guess you can't expect too much accuracy.

...for 60 nA and 10 megohm input resistance it is already 0.6 V. Too much if you want to check low voltage / low power microcontrollers. But it could be useful if you want to measure below 10 nA...

My entire purpose for buying something like the uCurrent Gold is to take nA measurements in conjunction with PIC MCUs. Let's say you want to calculate the internal impedance of a PIC input pin. We know the equivalent resistance is very high because it is the leg of a FET inside the PIC. But does it work out to be 10M? or 100M? or greater? Well, if we could put a precision Ammeter between say an external pull-up resistor and the PIC's input pin, we could check how much current flows through that pull-up into the PIC. I've tried it with my Fluke 8845A, but again, with both probes disconnected the Fluke shows 23nA or so. And taking measurements in a variety of ways indicates to me that current to be 100nA or less. So something like a uCurrent would help measure tiny current more accurately, which in turn can help find out the internal resistance of input pins, among other things.

But again, despite the fact I've been taking measurements on PIC circuits (12V input voltage dropped to 3V or 5V via voltage regulator), I've connected my Fluke meter incorrectly by accident several times through the years and have had to replace a few 400mA fuses. That's why if I buy a uCurrent I would need to mod the thing to protect against my own stupid mistakes. While rare, mistakes with a uCurrent would be much more costly than a costly Fluke 400mA fuse!

I didn't want to affect the circuit when powered on, but I figured it couldn't hurt to add back-to-back 1N5820 3A Schottky diodes across the inputs when the power is off. To do this, I added a jumper between SW2 pins 2B and 3B and put the diodes from SW2 pin 4B to the big trace connected to the minus side of R1. Note that this does nothing to protect the input when the power is on.

MLXXXp, can you give us some example situations, when the power is switched OFF, that would potentially destroy the uCurrent Gold in its stock condition (i.e., WITHOUT those two diodes)?

And for the rest of you uCurrent Gold users, have any of you ever destroyed your uCurrent when the power is OFF?

MLXXXp, can you give us some example situations, when the power is switched OFF, that would potentially destroy the uCurrent Gold in its stock condition (i.e., WITHOUT those two diodes)?

Even when the µCurrent is switched off, whatever the shunt resistor(s), R1, R2, R9, selected by the range switch will still be across the inputs. Any current flowing through the resistor(s) via the inputs, that causes the wattage to exceed the resistor ratings could potentially damage the resistor(s).

Also, even with the µCurrent switched off, the voltage across the shunt resistor(s) will also be impressed across the inputs of U1, but though R12, R5 and other circuitry, such that the actual current path is difficult to determine. The MAX4239 datasheet and any other documentation I've found doesn't say much about the maximum ratings for the inputs. It only states (VGND - 0.3V) to (VCC + 0.3V) but says nothing about how much current the inputs will tolerate when these voltages are exceeded. Therefore, it's difficult to predict the voltage across the shunt resistor(s) which would cause damage to U1 or other components in the path, but damage could occur if whatever voltage that happens to be were exceeded.

Strikes me that having back-to-back diodes might reduce the accuracy slightly on what is a very accurate instrument, by way of introducing a small parallel leakage. The opposite concern though, is that without diodes an overload may have decalibrated your µCurrent by burning out the shunt. You might not be aware of this for some time, and could be taking a whole load of incorrect measurements as a result.

If the diodes are heavily overloaded they will typically fail s/c, in which case you will be aware that there is a problem.

There is obviously a balance to be struck here, between having no protection and a very vulnerable instrument, to having the OTT protection most DMMs have, which raises the burden voltage to ridiculous levels.

I believe the 4xAAA batteries are powering the constant current load that the µCurrent is measuring, not the µCurrent itself.

The op amps in the µCurrent are rated 5.5V maximum, so I wouldn't power the µCurrent at anything higher than that. Also, if you increase the µCurrent's supply voltage, you run the risk of damaging the TPS3809 voltage supervisor IC due to increased LED current.